1. Field of the Invention
The present invention relates to a method of introducing magnetic anisotropy into a magnetic material and, more particularly, to a method of introducing magnetic anisotropy into a magnetic material used as a constituting element of various sensors.
2. Description of the Related Art
Various sensors (e.g., a torque sensor, a strain sensor, and a current sensor) using a magnetic material with magnetic anisotropy introduced therein as a constituting element have been developed. In these sensors, a change in external energy to be measured (e.g., a torque acting on a shaft, a strain applied to a flat plate, and a current flowing through a conductor) is converted to a change in magnetic characteristics (e.g., magnetic permeability) in a direction of an easy axis of magnetization of a magnetic material with magnetic anisotropy introduced therein, and the change in magnetic characteristics is detected, thereby detecting the magnitude of each of various external energies.
A uniaxial magnetic anisotropy must be introduced to a magnetic material constituting each of these sensors. Conventionally, the following methods are used to introduce uniaxial magnetic anisotropy into a magnetic material. However, these methods suffer from problems. A case will be exemplified wherein a uniaxial magnetic anisotropy is introduced into a magnetic material on the surface of a shaft constituting a torque sensor. The same applies to cases wherein uniaxial magnetic anisotropy is introduced into magnetic materials constituting other sensors.
(1) An annular magnetic member formed of an amorphous magnetic alloy ribbon is prepared in correspondence with the diameter of a shaft, and is subjected to a heat treatment to remove an internal stress. The magnetic member is then bonded while the shaft is twisted, and thereafter, the shaft is untwisted. In this method, the annular magnetic member must be prepared in advance in correspondence with the diameter of the shaft, and the shaft must be twisted, resulting in complex steps.
(2) A magnetic ribbon is subjected to a heat treatment and cooling in a magnetic field, thereby introducing uniaxial magnetic anisotropy. However, this method is very cumbersome, resulting in poor massproductivity. In addition, it is difficult to perform a heat treatment of an elongated magnetic ribbon. Therefore, the dimensions and shapes of magnetic ribbons are limited. When an amorphous alloy ribbon is subjected to a heat treatment, it becomes fragile.
(3) An amorphous alloy ribbon is bonded to a shaft by a hot isostatic pressing (HIP). Thereafter, a laser pulse is radiated on this ribbon now being crystalline to form a stripe pattern of amorphous phases (Published Unexamined Japanese Patent Application No. 63-280476). In this method, since the magnetic ribbon has a structure in which crystalline phases and amorphous phases are alternately aligned in the form of stripes, magnetic anisotropy can be introduced with this method, however, sufficient magnetic anisotropy cannot always be introduced.
As described above, when various sensors each using a magnetic material with magnetic anisotropy introduced therein as a constituting element are put into practical applications, a countermeasure against problems associated with introduction of magnetic anisotropy to a magnetic material is very important.